1. Field of the Invention
The present invention relates generally to the field of disc drive data storage devices, and more particularly but not by way of limitation, to an improved sealing means for preventing contaminants from entering a sealed head/disc environment.
2. Brief Description of the Prior Art
Rigid disc drive data storage devices of the type referred to as "Winchester" disc drives are well known in the industry. Such disc drives incorporate a number of disc-shaped members mounted on a spindle motor for rotation at a constant high rate of speed. These discs are coated with a thin magnetizable medium suitable for recording digital data which is stored on the discs in circular, concentric data tracks.
Data recording and retrieval is achieved by a read/write head formed either of an interrupted core element wrapped with wires or an analogous structure formed using thin-film deposition techniques. Electrical current introduced into the windings of the head induces a magnetic field in the core which extends significantly beyond the core in the area where the core is interrupted or the "gap". This magnetic field is used to magnetize the data medium to store data. When retrieving data, changes in the polarity of the magnetic flux in the medium induce small current spikes in the core windings, which are amplified and passed to read logic circuitry.
The actual read/write head mechanism is usually incorporated into a slider body that allows connection of the heads to an actuator used to move the head from track to track. The slider also serves another important purpose.
Before power is applied to the disc drive, the slider carrying the read/write head rests in direct contact with the surfaces of the discs. As the discs begin spinning, a thin layer of air is dragged along with the surface of the discs. At some point in the acceleration of the discs to operational speed, this thin layer of air interacts with ski-like features on the disc side of the slider, and the heads begin to "fly" above the disc surface on an air bearing. This means that during normal disc drive operation, the heads are not in direct contact with the discs, thus reducing wear and lessening the force necessary to move the heads from track to track.
In disc drives of the current technology, the "flying height", or distance between the head and the disc surface, is commonly less than ten microinches (0.000010 inch). A typical smoke particle or the thickness of body oil left in a finger print are on the order of ten times this size, and environmental dust is larger still. Thus, it is evident that cleanliness in the head/disc environment is of great concern in the disk drive industry.
For this reason, rigid disc drives typically are manufactured in a clean room environment and employ a sealed case, in which are enclosed the discs and the read/write heads, along with the actuator mechanicals to move the heads from track to track. Various forms of seals and gaskets have been used to insure the integrity of this sealed case, sometimes referred to also as a "bubble".
Typical disc drive designs incorporate a base casting to which are mounted all mechanical components, and a top cover attached to the base casting to complete the enclosure. Various forms of these two major components have been used:
1) a substantially planar base casting and a top cover having downward-extending sidewalls which mate with the perimeter of the base casting;
2) a pan-shaped base casting with upward-extending sidewalls and a generally planar top cover which sits on the edges of the base casting sidewalls, and;
3) a compromise configuration in which the base casting and top cover are both pan-shaped with sidewalls that meet to form the enclosed environment.
All of these configurations employ either edge-to-surface, or edge-to-edge contact.
Whichever of these configurations is used, the contact surfaces of these two parts never mate with sufficient perfection to provide sealing against the type of tiny contaminants noted above. Therefore, some sort of resilient gasket material is usually inserted between the base and top cover to insure adequate sealing, and the form which these resilient gaskets have taken is very diverse.
Die-cut foam rubber gaskets with pre-applied adhesives have frequently been used. Such gasket materials typically have voids and areas of varying density, so that an adequate seal can only be insured by compressing the foam to closely controlled tolerance. In the event that too little compression is achieved, the seal is suspect, while too much compression makes the gasket take a "set" and not reseal adequately if the "bubble" must be opened rework in manufacturing or repair at a later date.
Furthermore, commercially available foam rubbers sometimes include clay fillers which are left exposed when die-cutting is performed, or the material itself may flake and crumble when die-cut. Clearly, in such cases, the gasket becomes a source of possible contamination instead of a barrier against contamination as intended.
Various forms of molded gaskets have also been employed, again with varying degrees of success. One of the greatest drawbacks of molded gaskets is that such gaskets are typically very soft and difficult to handle and control in the manufacturing process. This makes proper alignment a tedious chore. Attempts to overcome this drawback have included molding the gasket with a U-shaped or H-shaped cross section. The edge of one or both of the mating components--or a specially formed raised lip--is then inserted in the grooves provided to facilitate alignment. Gaskets of this type are still subject to precise compression control as noted above, and the increased complexity of the mold adds significantly to the cost of the part.
Another drawback to molded gaskets is a phenomenon referred to as "outgassing". Most molded gaskets are formed of synthetic polymers which remain soft and pliable during use. However, these types of materials are subject to de polymerization--similar to evaporation--which frees molecules of the material within the sealed head/disc environment. While a single molecule would probably not be cause for concern, a large number of such molecules have been shown to re-polymerize and build up on head and disc surfaces to an extent large enough to interfere with the intended head/disc interface.
It is therefore desirable to produce a sealing method which overcomes these problems.